As shown in FIG. 8, a conventional ionization chamber 10 used for measurement of ionizing radiation has a charge collecting electrode 12 supported by an insulator 14, so that current (ionization current) collected at the charge collecting electrode 12 is measured. In FIG. 8, reference numeral 16 designates a high voltage source to form an electric field between the inner wall of the ionization chamber 10 and the charge collecting electrode 12.
The ionization current can be measured by various methods. In one example of the methods, as shown in FIG. 8, a micro-current meter 20 is connected directly to the charge collecting electrode 12. In another example, as shown in FIG. 9, a high resistor 22 is connected to the charge collecting electrode 12, and a voltage (potential difference) developed across the resistor 22 is measured with an electrometer (voltmeter) 24. In another example, as shown in FIG. 10, a capacitor 26 is connected to the charge collecting electrode 12, and, after the capacitor is reset by a reset switch 28, the variation of the voltage developed across the capacitor 26 is measured.
The above-described ionization chamber is extensively employed for measurement of external radiations, because it is stably sensitive to radiations, excellent in energy characteristic concerning X-rays and gamma rays, and low in manufacturing cost.
In addition, the ionization chamber may be used for measurement of the contamination of the air by radioactive materials or the concentration of radon by introducing the external air directly into the chamber. The ionization chamber un-sealed for the above purpose is called "ventilation type ionization chamber", hereafter.
As was described above, in the conventional ionization chamber, the charge collecting electrode 12 is supported with the ionization chamber by the insulator 14. Hence, the detection of weak radiations is limited by electrical noises such as the electrical leakage through the insulator 14 or its surface and piezo-electricity generated by the mechanical distortion of the insulator 14.
Therefore, in measurement of relatively weak radiations such as environmental radiations, it is often necessary to increase the sensitivity of the ionization chamber to decrease the influence of the insulator. Therefore, the volume of the ionization chamber is usually increased and/or the internal pressure of the ionization chamber is increased to several normal atmospheres. In addition, the electrical leakage of the insulator's surface is affected by humidity. Accordingly, it is necessary to maintain the humidity inside the ionization chamber low at all times. Especially for a ventilation type ionization chamber to measure the contamination of air by radioactive materials or the concentration of radon in the air, it is necessary to maintain the humidity inside the ionization chamber at low by using a desiccating agent.
In the ordinary natural circumference, the radiation level due to cosmic rays and natural radioactive materials is about 5 to 15 .mu.R/h. In the case of an air-tight ionization chamber, which is formed by using a material such as plastics whose atomic number is closed to that of air, and has a volume of one liter and one atmosphere inside it, the radiation of 10 .mu.R/h produces an ionization current of about 10.sup.-15 A.
In general, in the ordinary circumference, the concentration of radon in the air depends greatly on geographical conditions, housing conditions, ventilating conditions, weather conditions, etc. The average concentration of radon in a housing in Japan is estimated to be about 0.3 to 0.5 pCi/l. A caluculation shows that when one litter of air containing radon of 0.5 pCi/l is introduced into an ionization chamber, the expected ionization current will be about 10.sup.-15 A.
However, in general, the ionization current which can be measured stably for a long period of time with the conventional ionization chamber using the insulator is about 10.sup.-15 A or higher, and therefore it has been rather difficult to measure an ionization current of the order of 10.sup.-15 A.